Process for recovering valuable components from drilling fluid



March 18, 1969 PROCESS FOR RECOVERING VALUABLE COMPONENTS FROM DRILLINGFLUID R. F. BURDYN ET AL 3,433,312

ATTORNEY March 18, 1969 R, F, BURDYN ET AL 3,433,312

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3,433,312 PROCESS FOR RECOVERING VALUABLE COMPONENTS FROM DRILLING FLUIDRalph F. Burdyn, Dallas, and Murrell D. Nelson, Arlington, Tex.,assignors to Mobil Oil Corporation, a corporation of New York Filed June1, 1967, Ser. No. 642,740

U.S. Cl. 175-66 16 Claims Int. Cl. E21b 21/04; E21c 7/08 ABSTRACT OF THEDSCLOSURE This specification discloses a method of recovering and usinga valuable heavy solids phase and a valuable line solids phase from adrilling fluid being used to drill a well into subterranean formations.The method comprises the steps of: (a) running a portion of the drillingfluid through a particle segregator and effecting a separation of theportion of the drilling fluid into two streams; one, an underflowcontaining the heavy solids phase and, the second, a low densityeflluent containing an intermediate solids phase and the fine solidsphase, (b) returning the heavy solids phase to the drilling fluid beingcirculated, (c) subsequently treating the low density effluent toseparate the intermediate solids phase therefrom, and (d) employing thefine solids phase to control the properties of the drilling fluid beingcirculated. The subsequent treatment of step (c) preferably comprisesrerunning the low density effluent through a particle segregatoroperating under conditions effecting separation of the low densityetlluent into an underflow containing the intermediate solids phase andan effluent containing the fine solids phase. The treatment of step (c),however, may comprise adding a llocculant and subsequently separatingthe llocculated intermediate solids from the low density effluent.Furthermore, both (1) the step of rerunning the low density eflluentthrough the particle segregator and (2) the addition of a lluocculantand separation of a solids phase may be employed as the treatment ofstep (c).

Background of the invention This invention relates to a method ofrecovering valuable components from a drilling fluid and returning thesevaluable components to the drilling fluid being used to drill a wellinto subterranean formations.

As is well known, drilling fluids are employed when drilling holes intosubterranean formations. The drilling fluids have the primary purposesof: (l) cooling the bit, (2) carrying the cuttings away from beneath thebit, and (3) keeping the bottom of the borehole cleaned of thesecuttings. The most generally applicable drilling fluids and those towhich this invention relates are suspensions of solids in liquids,ordinarily called drilling mud or simply mud. The drilling fluidscontain large, dense, solid particles in order to impose adequatehydrostatic head to prevent blowouts, In blowouts, fluids occupying asubterranean formation, under pressure, pass into the wellbore, up thewellbore, and out at the surface. The large, dense, solid particles,usually barite, in the drilling lluid are referred to generally andherein as the heavy solids and a slurry thereof is referred to as theheavy solids phase. Further, the drilling fluids must have the propertyof thixotropy such that if circulation is interrupted, the cuttings onthe Way to the surface do not fall back into the well and stick the bitso as to prevent further rotation or movement thereof. Additionally, thedrilling fluids must have a low filter loss so the liquid component isnot lost into permeable subterranean formations, resulting in thick ltercakes that block the wellnted States Patent O Patented Mar. 18, 1969bore. Each of these properties requires the addition to the drillingfluid of valuable chemicals and, often, some small, low density clayparticles such as bentonite. The small, low density clay particles arereferred to generally and herein as line clay particles. Morever, thedrilling lluid must remain pumpable at high temperatures and have otherspecific characteristics Well known in the drilling fluid art. Thesecharacteristics also require the addition of other valuable chemicals tothe drilling fluid.

During the drilling operation, the drilling fluid picks up from thecuttings and from the subterranean formation additional intermediatesize, low density solids which adversely affect its properties. Theseintermediate size solids are referred to generally and herein asintermediate solids, and a slurry thereof is referred to as theintermediate solids phase. The intermediate solids are generallysiliceous and clayey solids. As the concentration of the intermediatesolids increases, treatment of the drilling fluid to maintain itsproperties becomes increasingly more expensive. Ultimately, aconcentration of intermediate solids is reached at which it becomeseconomically advantageous to discard a portion of the drilling lluid andreplace it with water, heavy solids, chemicals, and, where necessary,fine clay particles. The discarded portion of the drilling fluidcontains valuable heavy solids, valuable chemicals, and valuable fineclay particles.

Attempts have been made in the past to subject the drilling fluid tocentrifugal force in centrifuges to separate the solids and return apart of the solids to the drilling fluid. Single centrifuges, as well ascentrifuges in series, have been tried. These attempts have not beenaltogether successful because the thixotropy of the drilling fluidresists particle settling when the entire drilling fluid stream attainssubstantially the same velocity in the centrifuge chamber. However, aparticle segregator described in a copending application by Ralph F.Burdyn, entitled Method and Apparatus for Particle Segregation, Ser. No.397,497, filed Sept. 18, 1964, now U.S. Patent No. 3,400,- 819, issuedSept. 10, 1968, has made practical the separation of the heavy solidsphase from the drilling fluid.

Other attempts have been made to lluocculate the solids from thedrilling fluid and allow gravity settling. By its very nature, however,the drilling fluid resists such settling since it is thixotropic andprevents settling of the solids. Furthermore, because the drilling fluidcontains a large amount of solids having large surface areas,prohibitively large amounts of occulants are adsorbed on the solidsurfaces and make fiocculation prohibitively expensive as an initialtreatment.

No processes are presently available which successfully and economicallyextract from a portion of the drilling fluid the valuable heavy solidsphase and the desirable fine solids phase and ultimately return thesecomponents to the drilling fluid being circulated in a well whileeliminating the undesirable intermediate solids phase. The term finesolids phase is employed generally and herein to include: (1) linesolids made up of valuable, fine clay particles, and, unavoidably, somesmall silica particles and some smaller barite particles, (2) solutionof valuable chemicals, and (3) where present, oil, either as aseparateor an emulsiied phase.

Summary of the invention In accordance with the invention there isprovided a method of treating a drilling fluid, being circulated in awell being drilled into subterreanean formations, containing a valuableheavy solids phase, a detrimental intermediate solids phase, and avaluable tine solids phase including a valuable chemical solution whichrecovers the valuable phases and only discards the detrimental phase.The method comprises:

(a) Running a portion of the drilling fluid through a particlesegregator and effecting a separation of the portion of the drillingfluid into two streams; one, an underflow containing the heavy solidsphase and, the second, a low density effluent containing theintermediate solids phase and the fine solids phase,

(b) Returning the heavy solids phase to the drilling fluid beingcirculated,

(c) Subsequently treating the low density effluent to eliminatetherefrom the intermediate solids phase, and

(d) Employing the fine solids phase of the low density eflluent furtherin controlling the properties of the drilling fluid being circulated.

When a portion of the drilling fluid is run through a particlesegregator and separated into the two streams, the drilling fluid issubjected, within a chamber, to a pressure gradient sufficient to causea rate of flow thereof and to pass a part of the portion of the drillingfluid being treated through a centrifugal force field and permeableboundary generated by a perforated cylinder spinning at a ratesufficient to separate the drilling fluid, at the rate of flow, into theunderflow containing the heavy solids phase and into the low densityellluent containing the intermediate solids phase and the fine solidsphase.

Preferably, the treating performed in step (c) cornprises rerunning thelow density effluent through the particle segregator operating underconditions and in accordance with the basic formula describedhereinafter to separate the low density effluent into two streams, anunderflow containing the intermediate solids phase and an effluentcontaining the fine solids phase.

The employing of the fine solids phase of the low density eflluent instep (d) may comprise returning it to the drilling fluid beingcirculated or it may comprise recycling it to dilute any portion of thedrilling fluid being treated. Ordinarily, a part of the fine solidsphase will be returned to the drilling fluid being circulated and a partwill be used, with or without intermediate storage, to dilute a portionof the drilling fluid being treated, as described hereinafter.

While the pressure gradient may vary within the chamber in the particlesegregator, satisfactory results can be obtained by controlling thepressure differential between the inlet stream and the outlet streams.

Brief description of the drawings FIGURE 1 shows a schematicrepresentation of a drilling well employing the method of the invention.

FIGURE 2 is a longitudinal section taken through the particlesegregator.

FIGURE 3 is a transverse section taken along line 3 3 of FIGURE 2.

Description of specific embodiments The operation of a preferredembodiment of the invention and its environment is illustrated in thefollowing description with particular reference to FIGURE 1. Therein,well is being drilled into subterranean formation 12 while a drillingfluid is being circulated downward through drill string 14 and back upthe annular space 16 between the wall of the well and the drill string14. To be circulated downward through drill string 14, the drillingfluid is picked up from suction tank 18 by mud pump 20 and circulatedthrough hose 22 and into drill string 14. After the drilling fluid hasbeen circulated through the well and picked up cuttings from the bottomof the borehole, it returns to the surface and exits through pipe 24into said box 26, depositing the large cuttings onto the shale shakerscreen 27. The drilling fluid then flows from the sand box 26 intosettling tank 28 and thence to the suction tank 18 to complete thecycle. Intermediate mixing tanks 29, hoppers 30, and agitators 30a maybe employed to facilitate adding requisite chemicals and othercomponents.

In employing the invention, a portion of the drilling fluid is withdrawnfrom settling tank 28 by pump 31 and flowed through line 32 and openvalve 33 to particle segregator 34, frequently called a mud separator.Initially, additional water is pumped through line 36 to dilute thedrilling fluid to assist in making the separation in particle segregator34. As discussed hereinafter, a portion of the fine solids phase may beemployed as diluent when it is available. The amount of dilution may bedetermined empirically. Usually, 0.3 to 3.0 volumes of diluent pervolume of the portion of the drilling fluid being treated is adequate.However, no more than 2.0 volumes of water should be employed per volumeof drilling fluid being treated.

The portion of the drilling fluid passing to the particle segregator iscaused to flow within a chamber in the particle segregator under apressure gradient longitudinally along a centrifugal force field andpermeable boundary generated by a spinning 4perforated cylinderdescribed in more detail hereinafter. The pressure gradient is alsoimposed across the centrifugal force field and permeable boundary topass a portion of the drilling fluid therethrough. In this way, theheavy solids phase flows along the walls of the chamber and exits asunderflow from the particle segregator through line 38. Valve 40 ismaintained open during this portion of the cycle so the heavy solidsphase may be returned through line 42 to suction tank 18 and to thedrilling fluid being circulated.

The remainder of the portion of the drilling fluid passing through thecentrifugal force field and permeable boundary emerges as a low densityeflluent through line 44 and into storage tank 46. Valve 48 ismaintained open during this operation.

It has been found that employing a given, well-designed particlesegregator in this manner requires its operation only about 20 percentof the time. Stated otherwise, employing a good commercial particlesegregator at its designed circulation rate about 20 percent of the timeis ordinarily adequate to extract the heavy solids phase from theportion of the drilling mud which would otherwise be discarded in orderto retain the desired properties in the drilling fluid being circulated.

Accordingly, the particle segregator remains idle approximately 8()percent of the time. During this period of normal idleness, theremaining steps of the preferred embodiment of the invention may becarried out.

Valves 50, 52, 54, and 65, which are normally closed during theforegoing cycle, are opened. Valves 40, 48, 33, and 66 are closed.

The low density eflluent which has been stored in storage tank 46 ispicked up by pump 60 and circulated through line 62 to particlesegregator 34. The amount of water added through line 36 may be reducedby .closing down on valve 58. Frequently, it is possible to discontinueaddition of water altogether, in which case valve 58 is closedcompletely. When flowing the low density effluent to the particlesegregator, a lower volume throughput and lower pressure gradient areemployed, and the perforated cylinder is rotated at a faster rate ofrevolution to effect separation of the intermediate solids phase fromthe fine solids phase. Under these conditions, the intermediate solidsare unable to penetrate the centrifugal force field, flow along the wallof the chamber, and exit as underflow from the particle segregator, asdescribed in detail hereinafter. Thus, the intermediate solids phase isdischarged through line 38 and valve 54 to waste. Conversely, the finesolids phase penetrates the centrifugal force field and the permeableboundary adjacent the spinning perforated cylinder and flows, aseffluent phase, through line 44, line 64, and Open valves 52 and 65 toreturn to the drilling mud in suction tank 18.

In lieu of empolying a storage tank and rerunning the low densityeffluent, a bank of particle segregators connected in parallel may beinstalled to treat the low density eflluent in series with a primaryparticle segregator removing and returning the valuable heavy solidsphase from the drilling fluid. Ordinarily, such a bank of particlesegregators is not economically feasible, particularly in View of thefact that on most wells a single particle segregator is not employedfull time in removing the heavy solids phase and controlling theproperties of the drilling fluid. However, where such a bank of particlesegregators is employed to carry out a subsequent treatment of the lowdensity effluent by again subjecting it to the centrifugal force fieldand permeable boundary: (a) each particle segregator is operated atthroughput lower than that of the primary particle segregator and (b)the perforated cylinder therein is rotated at a rate higher than that ofthe primary particle segregator, as described hereinafter.

In another embodiment of the invention, the low density eflluent, havinghad a large part of its solids removed by the particle segregator, istreated by flocculating the solids therein to increase the averageparticle size of the suspended solid material and effecting separationof the particles.

The flocculation is effected by addition of suitable occulants to thelow density effluent. Suitable flocculants are well known. They includehigh molecular weight copolymers of vinyl acetate and of maleicanhydride, high molecular weight polyacrylamide polymers having from 1to 25 percent of their amide groups hydrolyzed, and high molecularweight polyethylene oxide polymers. The copolymers of vinyl acetate andof maleic anhydride are described by Lummus in an article, How to ReduceFine Mud Solids for Better Drilling, The Oil and Gas Journal, Mar. 22,1965, page 74. The partially hydrolyzed polyacrylamide is availablecommercially under the trade names of either Aerofloc or Separan. Thepolyethylene oxide is available commercially under the trade name ofPolyox.

A concentration of flocculant is employed which will effect adequatefluocculation, or aggregation, of particles to achieve the desiredlarger average particle size for subsequent separation. Where theflocculant is chemically incompatible with the drilling fluid beingcirculated, it is imperative that no more than about 2 percent by Weightof the flocculant be added to the low density effluent. Preferably, aconcentration of from about 0.01 to about 0.3 percent by weight isemployed.

The subsequent separation of the flocculated particles may be effectedby sedimentation under quiescent conditions, or by rerunning the lowdensity effluent through the particle segregator operated underappropriate conditions described hereinafter. By rerunning the lowdensity effluent is meant resubjecting the low density effluent to apressure gradient sufficient to pass a portion thereof through acentrifugal force field and permeable boundary effecting separation ofthe low density effluent into two streams; an intermediate solids phasedesignated underflow and a fine solids phase as effluent from theparticle segregator.

Where separation of the aggregated particles is to be by sedimentation,the low density effluent containing the flocculant is flowed into asuitable storage basin, which may be a tank, and allowed to remain forseveral hours or more under quiescent conditions to allow theflocculated solids to settle therefrom. After the settling has beeneffected to the desired degree, the supernatant fine solids phase can besiphoned or pumped therefrom and employed as desired; for example, beingreturned to the drilling fluid being circulated.

Conventional storage tank, piping, and pumping equipment are employed inflocculation, sedimentation, and subsequent use of the supernatant phaseand, hence, are not shown in the figures.

If the separation of the flocculated solids is to be effected byrerunning the low density effluent containing the flocculated solidsthrough the particle segregator, the procedure is carried out inessentially the same manner as described in the preferred embodiment ofthe invention.

A recycle procedure which we have found useful is to store a portion ofa fine solids phase for subsequent use in diluting the inlet stream intothe particle segregator. For example, instead of employing water throughline 36, the fine solids phase may be employed to dilute the portion ofthe drilling fluid flowed through line 32 into the particle segregator.The dilution with the fine solids phase is especially beneficial whenthe fine solids phase is composed primarily of chemical solution whichmay be prepared as described hereinafter. Also, the fine solids phasemay be employed to dilute the low density effluent from the particlesegregator before it is subsequently treated. This recycle of a portionof the fine solids phase is particularly advantageous since it preventsdilution of the heavy solids phase or the fine solids phase and reducesthe bulk volume being returned to the drilling fluid being circulated.

The recycle procedure is described with reference to FIGURE. 1. When thefine solids phase is being returned to the drilling fluid beingcirculated, valve 65 is open and valve 66 is closed. On the other hand,when the fine solids phase is Ibeing sent to storage tank 67 throughline 67a' valve 66 is open and valve 65 is closed. The ne solids phasemay be sent to storage and may be returned to the drilling fluid beingcirculated, in any proportion, by throttling flow through valve 65 andvalve 66.

When it is desired to dilute any low density effluent stored in tank 46,pump 67b can be employed to pump the fine solids phase into storage tank46 by closing valve 69, and opening valve 68 to dilute the low densityeffluent. Valve 68 is otherwise closed. Alternatively, pump 67b can pumpthe fine solids phase through line 69a 'when the fine solids phase isbeing employed to dilute the inlet stream to particle segregator 314. Inthis event, valve 69 is open or partly open. Otherwise, it is closed.Valve =58 is closed during such recycle to prevent dilution of the inletstream with water.

Infrequently, it lmay be desirable to clarify the light solids phase sothat only the valuable chemical solution be returned to the drillingfluid or stored for subsequent dilution of a portion of the drillingfluid being treated. lIn such event, the effluent passing out line 44from the particle segregator 34 following the rerunning of the lowdensity effluent may be routed to a third storage tank (not shown) and aflocculant added. After two treatments to remove the heavy solids andthe intermediate solids, the use of a flocculant to settle substantiallyall of the solids remaining in the fine solids phase becomeseconomically feasible.

The fiocculated particles are allowed to settle under quiescent storageconditions and the supernatant chemical solution pumped back into thedrilling fluid being circulated or used as diluent, with or withoutintermediate storage. The sediment, composed primarily of theflocculated particles, may be disposed of as waste slurry or employed asa slurry of fine solids in controlling the properties of the drillingfluid being circulated.

As mentioned previously7 care should be taken to ensure that any excessflocculant does not adversely affect the drilling fluid Iwhen the clearsupernatant chemical solution is pumped from the second storage tank andreturned to suction tank 18 to rejoin the drilling fluid beingcirculated.

Ordinarily, this clarification will not be necessary since the finesolids consist primarily of clay particles, and some silica particleshavin-g a size of less than about 2 microns, are beneficial, and neednot be discarded. In contrast, the intermediate solids consist of silicaand some clay particles having a size of from about 2 to about 10microns, adversely affect the drilling fluid, and should be discarded.

The segregation of particles within the drilling fluid is effected byimposing a pressure differential onto the liquid suspension such that aportion thereof flows as efiiuent through a centrifugal force field andpermeable boundary such as created by a perforated cylinder spinningwithin a chamber, and the remainder of the liquid suspension which doesnot penetrate the centrifugal force field and permeable boundary, at thepressure differential imposed, fiows as underflow from the chamber.Specifically, the particle segregation is effected in accordance withparticle size and density distribution. It is theorized that within thezone along the permeable boundary a relatively thin film is establishedin which laminar flow exists. 'In the zone between this thin film andthe inner wall of the chamber in which the separation process is carriedout, that is, in the space radially outward from the thin film along thepermeable boundary, a condition of high turbulence exists. This is to becontrasted with the condition existin-g in a conventional centrifuge`where the entire mass of drilling fiuid attains the same velocity asthe spinning container and thus tends to gel and resist movement of theparticles tending to be thrown to the outside by centrifugal force inmuch the same way as it resists settling of the particles in thewellbore.

Although the details of construction of a particular particle segregatorare not believed necessary, there are certain correlations between thesize of the various elements of a particle segregator which makepractical segregation of particles into the various phases. Thesecorrelations include: (l) the size of openings in relation to the sizeof the particles in the drilling fiuid to be treated and to the diameterof the cylinder being employed in setting up the centrifugal force fieldand the permeable boundary through which a portion of the drillingcfiuid is to be passed, (2) the total area of the openings relative tothe area of the cylinder, and (3) the length-to-radius ratio of thepermeable boundary and centrifugal force field formed by the rotatingperforated cylinder.

The elements in the particle segregator are illustrated in FIGURE 2.Therein, outer casing 70 is united with a pair of opposed end members 72and 74 by cap screws 76 to form a pressure-tight chamber. While thecasing is shown as being substantially cylindrical in shape, it -rnayhave any other desired configuration. Positioned within the chamber is arotatable, hollow cylinder 80 provided with a plurality of openings 82extending through the cylindrical wall and distributed oversubstantially the entire surface thereof. `Openings 82 are preferablyevenly distributed over the surface of the cylinder. Cylinder 80 issupported by a pair of solid end plates `84 and 86. End plate 84 issecured to a driven shaft 88 while end plate 86 is secured to a hollowshaft 90. Suitable rotary sealing units, generally designated byreference numeral 92, surround driven shaft 88 and hollow shaft 90 toprevent leakage from the chamber within casing 70. The sealing units mayinclude means for introducing a sealing fluid under pressure greaterthan that in the chamber into the annulus between each shaft and itsrespective end member. Suitable bearings, generally designated byreference numeral 94, are of the ball-bearing type and are fastened bycap screws 95 or other suitable means to the end members 72 and 74 topermit free rotation of shafts 88 and 90. A rotating union 96 isthreaded to the outer end of shaft 90 and is supported in a coupling 98.

A discharge pipe 100, provided lwith apertures 102, is supported withinthe cylinder 80 with one end thereof in communication with the hollowshaft 90. A plurality of vanes 104, illustrated in FIGURE 3, extendradially outwardly from pipe 100 to cylinder 80. The varies 104 are notphysically attached directly to cylinder 80 but are supported by annularbraces 105.

Cylinder 80 is rotated at predetermined speeds by suitable means (notshown) to generate the previously mentioned centrifugal force field andpermeable boundary through which a part of the drilling fluid beingtreated is fiowed and 'which in turn imparts centrifugal force to theportion of the drilling fluid immediately surrounding the cylinder.

'Ihe drilling fluid to be treated enters the fluidtight chamber at entryport 107, shown in FIGURE 3. Port 107 is mounted off-center and is notillustrated in FIG- URE 2. It is not necessary that port 107 be mountedoffcenter. The portion of fluid and of fine solids flowing throughperforations `82, exits as effluent from the interior of cylinder by wayof apertures 102 in pipe 100, hollow shaft 90, rotating union 96, anddischarge conduit 110. The subsequent handling of this effluent isdescri-bed under the specific steps elsewhere.

The remainder of the fluid, including the solid particles having a sizeand weight such that their inward flow momentum due to the pressuredifferential is overcome by centrifugal force, exits as underfiowthrough a discharge conduit 112 in casing 70 at a controlled rate.Further handling of this underflow passing through discharge conduit 112is discussed with respect to different steps elsewhere.

The openings 82 should be at least several times as large as the largestparticles in the drilling fluid being treated to prevent severalparticles collectively forming a bridge over an opening and effectivelyclosing off the opening. The forming of a bridge over an opening iscalled bridging It is possible for each of the openings to be severaltimes as large as the largest particles in the drilling fluid 4beingtreated because the separation process is not in any way dependent upona filtering or screening action by the openings -in the rotatablecylinder. lf the openings are small enough to permit bridging, theapparatus can become inoperative due to particles clogging the openings.Successful operations have been performed where the diameters of theperforations range from as small as 3%.; inch to as large as 5/8 inch.The size of the openings which can be usefully employed in a givenparticle segregator has been found to be contingent upon the diameter ofthe rotating cylinder. The ratio of the diameter of the openings to thediameter of the cylinder is preferably between about 0.01 and about 0.1.At ratios below the above-mentioned lower limit, the possibility ofbridging of the openings is increased. At ratios above the upper limitof the preferred range, the efficiency of the separation is decreasedand less barites are recovered or, correspondingly, less intermediatesolids are removed. It is theorized that the decreased efficiencyresults from the larger openings causing irregularities in the boundarylayer formed at the surface of the rotating cylinder.

The total area of openings should be from about 5 to 30 percent of thetotal area of the cylinder, exclusive of the end portion in order toform a boundary which is instantaneously permeable over about 5 to about30 percent of its area. Preferably, the area of the openings and hencethe area of instantaneous permeability of the boundary will be betweenabout 15 and 25 percent of the total cylindrical surface area of thecylinder. While operation outside of these ranges will enable someseparation to be effected, the amount of separation achieved is notsufficient to provide a process that is advantageous over knownseparating processes.

For good operating efficiency, it is preferred that the length-to-radiusratio of the substantially cylindrical, permeable boundary be at least 6and preferably about 16. Such ratios are preferred since, for a cylinderof given volume, the horsepower required to rotate a perforated cylinderto generate the permeable boundary is directly dependent upon the fourthpower of the radius but only the first power of the length. It will thusbe appreciated that the power requirements of a cylinder having alengthto-radius ratio in the above-mentioned range will have greatlydecreased power requirements when compared, for example, to a segregatorof the same throughput wherein the length and diameter of the rotatablecylinder are approximately equal.

It has also been found that for best operation the fiow rate of thefluid mixture to the segregator and the flow rate of the streamsdischarging from the segregator should be maintained fixed relative tothe rate of rotation. This may be achieved by the control means andpumps well known to the art and particularly discussed in applicationSer. No. 397,497 mentioned previously.

The uid mixture should also be supplied to the particle segregator at aconstant pressure within the range of 15 to 45 p.s.i.g. If, on the otherhand, the fluid mixture is supplied by means of a hydrostatic head, aconstant rate of flow of the various streams relative to the rotation ofthe cylinder is not achieved and a less efficient particle separationresults. It will be apparent that, while remaining within the preferredranges of the correlations referred to above, a fairly -wide range ofdesigns is possible. For example, in a particular test model wherein theperforated cylinder was 51/2 inches long, had an outside diameter of11/2 inches, providing an effective area of 26 square inches, and theperforations were inch in diameter, no appreciable variation inefficiency of operation was detected where the cylinder was provided-with 200 perforations as compared with a cylinder of the same dimensionhaving a total of 1,580 perforations. However, when the total number ofperforations in the cylinder was increased to 3,300 so that the totalarea of the openings was about 39 percent of the area of the cylinder,the efficiency of the apparatus fell from about 96 percent toapproximately 86 percent.

The relationship between the flow rate of drilling fluid and thecentrifugal force field to which a part of the drilling fluid must besubjected to effect particle segregation in accordance withpredetermined values of size and density is defined by the formula:

Q=the rate of flow of the eflluent stream,

R=the radius of the centrifugal force field and a substantiallycylindrical permeable boundary which for design purposes may be assumedto be the radius of a perforated cylinder which will be rotated within achamber,

L=the length of the centrifugal force field and substantiallycylindrical permeable boundary which for design purposes may be assumedto be the length of the perforated portion ofthe perforated cylinder,

w=a factor indicative of the magnitude of the centrifugal force fieldactive to restrict movement of particles to the interior of thesubstantially cylindrical permeable boundary, and which for designpurposes may be assumed to be the angular speed of the perforatedcylinder in radians per second,

Ap=the difference in density between the particles to be separated landthe effluent stream,

d*=the size at which particle segregation is to be effected; that is,the minimum size of the particles to be separated as underflow, or themaximum size of particles having a density equal to that of theparticles to be separated, to pass out with the eflluent, and

L=the viscosity of the effluent stream.

Equation 1 can be expressed in a more convenient, equivalent formulaemploying engineering units as follows:

when:

Q is given in gallons per minute,

R is given in inches,

L is given in inches,

N is the factor w when that factor is in revolutions per minute insteadof radians per second,

Ap is given in pounds per cubic foot,

d* is given in microns, and

n is given in centipoises.

In accordance with Equation 2, heavy barite particles that are in excessof 5 microns in size may be separated from an eflluent stream having adensity of 75 pounds per cubic foot under the following conditions:

Q= 10 gallons per minute,

R=2.85 inches,

L=42.75 inches,

N= 1060 revolutions per minute (r.p.m.), Ap= 193 pounds per cubic foot,and

/t=3 centipoises.

Still approximately in accordance with Equation 2 but disregardingdecreasing density and viscosity of the effluent stream, the low densityeflluent may be pumped from the storage tank and rerun to the particlesegregator to effect separation of barite particles down to 1 micron insize. Rerunning the low density efiluent at the flow rate Q initiallyemployed would require an increase in the rate of revolutions (rpm.) bya factor of 5. Since such increase in r.p.m. would require severe designcriteria, a more practical approach is ordinarily Iadvisable where Q maybe cut by a factor of 5 and the r.p.m. increased by a factor of \/5 to2,360 and the desired separation effected.

In rerunning the low density effluent, the primary objective is toeliminate therefrom the silica yand clay particles having sizes fromabout 2 microns to about l0 microns in effective dimensions. Since thedifferential density of the silica and clay particles is approximatelyone-half that of the barite, this separation will unavoidably separatethe barite particles of size between about 1.4 and about 7.1 microns.These intermediate solids are undesirable and are separated as underflowfrom the particle segregator.

A particle segregator having a perforated cylinder 2.85 inches in radiusand 42.75 inches in length can be run as high as 3,000 r.p.m. withoutexcessive mechanical difficulties and with reasonable power requirementswhen treating the low density eflluent. With an approximation of 97.3pounds per cubic foot difference in the density of the particles beingseparated and the density of the eflluent from the particle segregator,which effluent has a viscosity of about 2 centipoises, Equation 2reduces to a simplified version, which is approximately accurate, ofQ=2.42d2. Stated otherwise, silica and clay particles larger than about2 microns in size may be separated as an intermediate solids phasecomprising the underflow from the low density efluent when the lowdensity effluent is caused to flow, under a pressure gradient suflicientto afford an eflluent flow rate of up to 9.6 gallons per minute, throughthe cylindrical, permeable boundary created by the perforated cylinderrotating at 3,000 r.p.m.

On the other hand, when the drilling fluid containing the bariteparticles is being treated with the same particle segregator, theperforated cylinder can be run as high as 2,300 r.p.m. with about thesame power requirements. When the perforated cylinder is run at 2,300r.p.m., the particle segregator will effect a separation of bariteparticles above about 5 microns in size yfrom the low density effluentat flow rates Q up to about 47 gallons per minute. As can be seen from acomparison, the flow rate can be much higher during this initial flowthrough the particle segregator than in the subsequent recycle of thelow density eflluent. In the examples described, the difference factoris about five times. It may run from about three to yabout eight timesas high depending on the initial and final separation conditions sought.

Having thus described the invention, it will be understood that suchdescription has been given by way of illustration and example and not byway of limitation. The appended claims define the scope of theinvention.

What is claimed is:

1. A method of treating a drilling fluid being circulated in a wellbeing drilled into subterranean formations and containing a valuableheavy solids phase, a detrimental intermediate solids phase, and avaluable fne solids phase including a valuable chemical solution, whichcomprises the steps of:

(a) running a portion of said drilling fluid through a particlesegregator and effecting a separation of said portion of said drillingfluid into an underflow containing said heavy solids phase and into alow density ellluent containing said intermediate solids phase and saidline solids phase, said separation comprising subjecting said portion ofsaid drilling fluid, within a chamber, to a pressure gradient sufficientto cause a rate of flow thereof, and to pass a portion thereof through acentrifugal force field and permeable boundary generated by a perforatedcylinder spinning at a rate sullicient to separate said portion of saiddrilling lluid at said rate of ow into said underllow containing saidheavy solids phase and into said low density effluent containing saidintermediate solids phase and said line solids phase,

(b) returning said heavy solids phase to said drilling fluid beingcirculated,

(c) subsequently treating said low density eilluent to separatetherefrom said intermediate solids phase, and

(d) employing said dine solids phase of said low density ellluentfurther in controlling the properties of said drilling fluid beingcirculated.

2. The method of claim 1 wherein said treating of step (c) comprisesrunning said lo-w density eflluent through a particle segregator so thatsaid low density effluent is subjected, within a chamber, to a pressuregradient sufficient to cause a rate Iof flow thereof, and to pass aportion thereof through a centrifugal force lleld and premeable boundarygenerated by a perforated cylinder spinning at a rate sutlicient toseparate said low density effluent at said rate of flow into anunderflow containing said intermediate solids phase, and into aneflluent containing said fine solids phase including said chemicalsolution.

3. The method of claim 2 lwherein a llocculant is added to said effluentcomprising sa-id fine solids phase after said intermediate solids phasehas been separated from said low density eflluent, said effluent isstored under quiescent conditions to effect gravity settling of saidfine solids and form, as a supernatant liquid, said chemical solution,containing substantially less fine solids, and employing said chemicalsolution lfuther in controlling the properties of said drilling fluidbeing circulated.

4. The method of claim 2 wherein said step (a) and said step (c) towhich said drilling fluid and said low density effluent are subjected,respectively, comprises:

(a) imposing within an enclosing chamber a pressure differential acrossa substantially cylindrical boundary permeable to said drilling fluid toeffect passage of a portion of said drilling fluid and said low densityeffluent inwardly toward said boundary at a rate dependent upon saidpressure differential,

(b) creating a zone of rotation at said boundary to impose a centrifugalforce field upon said drilling fluid and said low density eflluentpassing through said boundary, the magnitude of said centrifugal forcefield being `dependent upon the rate of rotation,

(c) regulating said rate of rotation relative to said pressuredifferential so that the inward flow momentum of each of the particlescomprising, respectively, heavy solids in said drilling fluid andintermediate solids in said low density eflluent having a size anddensity above predetermined values will be overcome and said particles1will not pass through said boundary but the -fine solids in saiddrilling lluid and in said low density etlluent having a size and adensity below said predetermined values will pass through said boundary,

(d) withdrawing from said chamber outside olf said cylindrical boundary,liquid together with, respectively, said heavy solids and saidintermediate solids, and

(e) withdrawing from inside said cylindrical boundary,

liquid which has passed through said boundary together with said finesolids.

5. The method of claim 2 wherein said step (a), and said treating ofstep (c) to which Said drilling fluid and said low density eflluent,respectively, are subjected is achieved by:

(a) introducing, respectively, said drilling fluid and said low densityeffluent into a chamber,

(b) subjecting, respectively, said drilling fluid and said low densityeffluent within said chamber to a pressure differential across asubstantially cylindrical boundary created by a spinning perforatedcylinder and instantaneously permeable to said drilling fluid over fromabout 5 percent to about 30 Ipercent of its area without bridging of thelargest of solid particles in said drilling fluid to effect passage,respectively, of said drilling fluid and said low Vdensity effluentinwardly toward said boundary at a rate dependent upon said pressuredifferential,

'(c) creating a centrifugal -force field, whose magnitude is dependentupon the rate of rotation of said spinning perforated cylinder, at saidboundary to impose a centrifugal force upon, respectively, said drillinglluid and said low density eflluent passing through said boundary.

(d) regulating said rate of rotation relative to said pressuredifferential so that the inward flow momentum of each of the solidparticles comprising, respectively, heavy solids in said drilling fluidand intermediate solids in said low density etlluent having a size and adensity above predetermined values will be overcome and said solidparticles will not pass through said permeable boundary but the finesolids in said drilling fluid and in said low density effluent having asize and a density below said predetermined values will pass throughsaid boundary, l

(e) withdrawing from said chamber outside of said cylindrical boundary,respectively, said heavy solids phase and said intermediate solidsphase, and

(f) withdrawing from inside said cylindrical boundary said fine solidsphase.

6. The method of claim 5 wherein said pressure differential and hencesaid rate of flow of effluent, said rate of rotation and dimensions ofsaid perforated cylinder, said predetermined values of said size andsaid density of said heavy solids in said drilling fluid, and saidpredetermined values of said size and said density of said intermediatesolids in said low density effluent are determined in accordance withthe following equation:

where: K=15.97 X10-12, Q=the rate of llow of the effluent stream ingallons per minute, R=the radius in inches of said centrifugal forcefield and said substantially cylindrical permeable boundary, which isessentially the radius of a perforated cylinder which will be rotatedwithin a chamber, L=the length in inches of said centrifugal torce lleldand said Substantially cylindrical permeable 13 of said intermediatesolids and said effluent of said tine solids phase,

d*==the size in microns at which particle segregation is to be effected;that is, the minimum size of, respectively, said particles of said heavysolids and said particles of said intermediate solids, or the maximumsize of particles having a density equal to that o-f said particles ofsaid heavy solids and said particles of said intermediate solids to passout with the efliuent stream comprising, respectively, said low densityeffluent and said iine solids phase, and

,u=the viscosity in centipoises f, respectively, said low density eiuentand said tine solids phase.

7. The method of claim wherein said pressure differential and said rateof rotation ,are adjusted to eiect separation of heavy solids having asize of more than 5 microns and a density comparable to that of baritefrom said drilling fluid, and of said intermediate solids having a sizeof about 2 to 10 microns and a density about equal to that of silicafrom said low density etiuent.

8. The method of claim 7 wherein said intermediate solids have a size ofabout 2 microns.

9. The method of claim 5 wherein said substantially cylindrical boundaryis instantaneously permeable to the passage therethrough of liquid overabout to 25 percent of its area without bridging.

10. The method of claim 1 wherein said treating of step (c) comprisesadding a iiocculant and subsequently separating the occulatedintermediate solids from said low density etiluent.

11. The method of claim 10 wherein said subsequent separation of saidintermediate solids from said low density effluent is performed bystoring said low density eliuent and said tiocculant under quiescentconditions to effect sedimentation of occulated intermediate solids andpumping the supernatant iine solids phase from the sediment of saidintermediate solids.

12. The method of claim 11 wherein said subsequent separation of saidintermediate solids from said low density euent is effected by rerunningsaid low density etliuent through said particle segregator so that saidlow density effluent containing occulated intermediate solids issubjected, within a chamber, to a pressure gradient sufcient to cause arate of dow thereof and to a centrifugal force field and permeableboundary generated by a perforated cylinder spinning at a ratesuiiicient to separate said low density effluent containing occulatedintermediate solids into underflow comprising said intermediate solidsphase and into an eiuent comprising said fine solids phase.

13. The method of claim 1 wherein said employing of said line solidsphase of step (d) comprises returning said ine solids phase to saiddrilling uid being circulated.

14. The method of claim 1 wherein said employing of said tine solidsphase of step (d) comprises recycling a portion of said line solidsphase to dilute at least a portion of said drilling fluid being treated.

15. The method of claim 1 wherein said employing of said fine solidsphase of step (d) comprises both returning said fine solids phase toSaid drilling fluid being circulated and recycling a portion of saidtine solids phase to dilute at least a portion of said drilling uidbeing treated.

16. The method of claim 1 wherein said running a portion of saiddrilling uid through a particle segregator and effecting said separationin step (a) comprises:

subjecting said portion of said drilling uid, within a chamber, to apressure gradient suicient to cause a rate of flow thereof and to pass aportion thereof through a centrifugal force eld and permeable boundarygenerated by a perforated cylinder spinning at a rate suflicient toseparate said portion of said drilling iiuid at said rate of ow into anunderflow containing said heavy solids phase and into a low densityeiiiuent containing said intermediate solids phase and said fine solidsphase, and

wherein said treatment in step (c) comprises:

running said low density effluent through a particle segregator so thatsaid low density etiiuent is subjected, within a chamber, to a pressuregradient suiiicient to cause a rate of iiow thereof, and to pass aportion thereof through a centrifugal force field and permeable boundarygenerated by a perforated cylinder spinning at a rate sufficient toseparate said low density eiiiuent at said rate of flow into an underowcontaining said intermediate solids phase, and into an eiiluentcontaining said tine solids phase including said chemical solution.

References Cited UNITED STATES PATENTS 2,225,973 12/ 1940 Brown et al.175--66 X 2,870,990 1/ 1959 Bergey 175-66 2,919,899 l/1960 Marwil et al.-66 3,016,962 1/ 1962 Lummus et al. 175-66 OTHER REFERENCES Anon., Howto Reduce Fine Mud Solids for Better Drilling, The Oil and Gas Journal,Mar. 22, 1965 (pp. 74-77).

Burdyn, A New Device for Field Recovery of Barite Society of PetroleumEngineers Journal, June 1965, (pp. 10U-108).

STEPHEN I. NOVOSAD, Primary Examiner.

UNITED STATES PATENT GEEICE CERTIFICATE OF CORRECTION Patent No.3,433,312 March 18, 1969 Ralph F. Burdyn et al.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 3, line 69, "said box 26" should read sand box 26 Column 5 line38, "fluocculation" should read flocculation Column ll, line 32"premeable" should read permeable line 45., "futher" should read furtherline 66, before "density" insert a Column l2, line 26, "boundary. shouldread boundary, Column 13, line l9, before "10 microns" insert about line38, "The method of claim ll" should read The method of claim l0 Column14, line 43, "2,919,899 should read 2 ,919,898

Signed and sealed this 31st day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer WILLIAM E. SCHUYLER, JR.

Commissioner of Patents

